I had the same problem with my R91; likely cause is that the threaded tube that holds the center pin is loose. To fix it, remove the center pin, stick a small flathead screwdriver into the 510 connection, and tighten down the tube. Make sure to just tighten snugly; this tube also holds the build deck in place, and I've broken the insulator before by tightening it too much. It helps to hold the build posts in place while doing this so that the positive post doesn't go sideways.
Crap, I broke my insulator. Any ideas where I can get a replacement?